Surface light source device and display

ABSTRACT

Overlapping first and second light guide plates have incidence faces located opposite to each other. Each light guide plate has an emission restraint region, an emission gradually-increasing region and an emission promotion region arranged in order from incidence face toward distal end face. In each emission gradually-increasing region, emission light quantity increases gradually away from each emission restraint region toward each emission promotion region. The emission gradually-increasing regions of the light guide plates overlap each other. The emission promotion region of the second light guide plate overlaps with the emission restraint region of the first light guide plate while the emission restraint region of the second light guide plate overlaps the emission promotion region of the first light guide plate. Light from LEDs is limited by windows. Color-mixing of the LED-light occurs in each emission restraint region to become whitened light gaining emission intensity in the emission promotion regions.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a surface light source device and to a display employing the surface light source device. The present invention can be applied to displays in devices such as car navigation devices, video cameras, digital still cameras, electronic pocket notebooks, portable telephones, portable mobile terminal devices, personal computers, or LCD-TV sets.

The present invention can be also applied to surface light source devices for backlighting a member-to-be-illuminated (such as LCD-panel) in the above displays, or to surface light source devices for backlighting an advertising panel or the like. The present invention is suitably applied to, for example, surface light source devices of a type such that white illumination output light is provided by adopting LEDs emitting light of a plurality of colors as primary light source, and displays (such as car navigation devices, video cameras) employing the same,

2. Related Arts

Known devices such as car navigation devices have employed displays each consisting of a LCD-panel and a surface light source device for backlighting the LCD-panel. In general, surface light source device include a thin light guide plate and a rod-like fluorescent lamp (primary light source) disposed along a side face (incidence face). Light (primary light) from the fluorescent lamp enters into the light guide plate through the incidence face, becoming inner propagation light. Emission is provided by an emission face generally vertical to the incidence face on the way of inner propagation.

In general, However, employment of fluorescent lamp as primary light source brings problems like (1) to (3) below.

(1) Environment is affected when fluorescent lamps are disposed because they contain mercury.

(2) Electric power consumption is large.

(3) A driving circuit including an inverter is required.

Under such situation, it has been proposed, as disclosed in Patent Document 1 noted below, to employ a so-called “white LED” instead of fluorescent lamp, and white LEDs have been put into practical use. A white LED is a light emitting means outputting white light, which has been realized because blue LEDs have been put in practical use.

If a white LED(s) is(are) employed as a primary light source of a surface light source device, the above problems (1) to (3) are overcome.

On the other hand, displays with high colour fidelity, such as plasma display have been realized recently. Thus displays of a type such that a member-to-be-illuminated like lcd-panel is backlighted by a surface light source device are also comes to be required to have a high displaying quality, especially color displaying performance (color fidelity).

It is noted that “white LED” is a light emitting source generating light of a color like white by combining light of a blue LED with light of fluorescent substance. Now such a method of generating white light is called “white LED method” for the sake of convenience.

According to another known method, white light is generated by mixing light outputted from LED elements of R (red), G (green), B (blue) disposed so that number of LED element of each color is one or more. Now such a method of generating white light is called “LED light color mixing method” for the sake of convenience. It is known that LED light color mixing method is superior in color fidelity as compared with white LED method.

FIGS. 15 and 16 show an example of outlined structure of surface light source device employing LED light color mixing method. Such structure is disclosed in Patent Document 2 noted below.

Referring to FIGS. 15 and 16, surface light source device 102 includes two light guide plates 103, 104, a first group of LEDs and a second group of LEDs.

Light guide plates 103, 104 have one shape, being disposed so that the two are piled up vertically. Each group of LEDs are consist of three LEDs 105, 105, 105 of R, G and B, respectively. The first group of LEDs are disposed opposite to the right side face of upper side light guide plate 103 and the second group of LEDs are disposed opposite to the left side face of lower side light guide plate 104.

Light from the first group of LEDs 105 is color-mixed mainly within light guide plate 103, being emitted from emission face 107. Light from the second group of LEDs 105 is color-mixed mainly within light guide plate 104, being emitted from emission face 106 and then emitted from emission face 107 via light guide plate 103.

In such a way, light from the first and second groups of LEDs 105 becomes illumination output light emitted from emission face 107 of light guide plate 103 overall.

-   -   Patent Document 1; Tokkai-Hei 10-242513     -   Patent Document 2; Tokkai 2006-100102

Further saying, a still another art as shown in FIG. 17 is known. Referring to FIG. 17, a plurality of (a lot of) LED groups are disposed along incidence face(s) 108 of upper and lower light guide plate(s) 103 (104). This enables light guide plates 103 (104) to be supplied with enough quantify of light.

However, the above-described prior arts have a problem which has been not solved yet. Brief explanation on this problem is given below by using the example shown in FIG. 17.

Now attention is paid to light emitted from LEDs 10s which are included in many LEDs 105 and located at both ends respectively. A hatched part shown gives about a half of such light. Much of the light of the hatched part males any route of the followings after entering into light guide plate 103 (104).

(a) A route along which light is emitted from emission face 107 of upper light guide plate 103 after being inner-reflected (turning up of light path) by side face 110, 110.

(b) A route along which light is inner-reflected (turning up of light path) by a frame or the like, not shown, after being emitted from side face 110, 110 of light guide plate 103 (104). Further, this light enters again into light guide plate 103 (104), then being emitted from emission face 107 of upper light guide plate 103 together with light from LEDs located at both sides respectively.

Next, for study the whole primary light, numerals (I) to (V) depending on location in the arrangement are labeled on LEDs constituting a LED group as shown in FIG. 17. Further, provided is that light emitted from each of these LEDs 105, (I) to (V) enters into light guide plate 103 at the same entrance efficiency.

Under this condition, measurement point B on the emission face can receive light obliquely emitted from LEDs (I), (II), (IV), in addition to light from LED 105 (III).

Therefore, emission at measurement point B becomes white light easily through undergoing colour-mixing of light components of a plurality of colours.

On the other hand, light emitted from LEDs 105 (IV) and (V) hardly reach measurement point A. As a result, the proportion of light component reaching measurement point A from LED 105 (I) to the total light components reaching measurement point A becomes relatively large. In a similar manner, the proportion of light component reaching measurement point C from LED 105 (V) to the total light components reaching measurement point C becomes relatively large.

As understood from the above discussion, although a large-are center potion in emission face 107 can be prevented from showing colour unevenness and coloring, colour(s) of light from LEDs 105, 105 located at both ends of a row-like arrangement of LED group is(are) emphasized.

This emphasized color(s) of light from LEDs 105 cause(s) emission from emission face 107 of upper light guide plate 103 to be affected by colour unevenness or coloring locally. Colour unevenness or coloring locally tends to appear, in particular, in the vicinity of side faces 110, 110 of rectangular surface-like illumination emission.

DISCLOSURE OF INVENTION

An object of the present invention is to prevent a surface light source device of the above type from having illumination output light (output light of the surface light source device) affected by local colour unevenness which could emerge due to point-like light sources of both sides of a point-like light source group.

According to the present invention, in cases where white light is generated by colour-mixing of light respective from point-like light sources, local colouring (i.e. outputting of non-white light) is avoided form emerging due to the same cause. In addition, still another object of the present invention is to provide a display of a superior displaying performance by utilizing the surface light source device free from colour unevenness and colouring of light.

In the first place, the present invention is applied to a surface light source device comprising a first light guide plate, a first group of point-like light sources including at least three point-like light sources of emission colours different in emission colour, a second group of point-like light sources including at least three point-like light sources of emission colours different in emission colour, said first light guide plate having a first incidence face provided by a side end face, a first distal end face opposite to said first incidence face, a first emission face provided by a major face and a first back face opposite to said first emission face, said second light guide plate having a second incidence face provided by a side end face, a second distal end face opposite to said second incidence face, a second emission face provided by a major face and a second back face opposite to said second emission face, said first light guide plate and said second light guide plate overlapping each other so that said first emission face and said second back face are disposed face to face across a low-refractive-index-layer, which is interposed between them and has a refractive index lower than those of said first light guide plate and said second light guide plate, and said first incidence face and said second incidence face are located at different sides with respect to said first light guide plate and said second light guide plate, point-like light sources belonging to said first group of point-like light sources being disposed along said first incidence face at a predetermined interval. and, point-like light sources belonging to said second group of point-like light sources being disposed along said second incidence face at a predetermined interval.

According to a basic feature of the present invention, said first light guide plate is provided with a first emission restraint region, a first emission gradually-increasing region and a first emission promotion region arranged in order away from said first incidence face toward said first distal end face, and,

said second light guide plate is provided with a second emission restraint region, a second emission gradually-increasing region and a second emission promotion region arranged in order away from said second incidence face toward said second distal end face.

Further, said second emission restraint region overlaps with said first emission promotion region in a thickness-direction of said first light guide plate and said second light guide plate, and said second emission promotion region overlaps with said first emission restraint region in the thickness-direction of said first light guide plate and said second light guide plate, and said second emission gradually-increasing region overlaps with said first emission gradually-increasing region in the thickness-direction of said first light guide plate and said second light guide plate, wherein emission promotion ability in said first emission gradually-increasing region increases gradually and smoothly away from said first emission restraint region toward said first emission promotion region, and emission promotion ability in said second emission gradually-increasing region increases gradually and smoothly away from said second emission restraint region toward said second emission promotion region.

Still further, a first limiting means is disposed between said first incidence face and each of point-like light sources which are both-end-located point-like light sources of said first group of point-like light sources and are located at both ends of said first group of point-like light sources respectively, in order to limit light quantity incident to said first incidence face from said both-end-located point-like light sources of said first group of point-like light source, and, a second limiting means is disposed between said second incidence face and each of point-like light sources which are both-end-located point-like light sources of said second group of point-like light sources and are located at both ends of said second group of point-like light sources respectively, in order to limit light quantity incident to said second incidence face from said both-end-located point-like light sources of said second group of point-like light sources.

It is noted that at least three point-like light sources belonging to said first group of point-like light sources may have emission colors which generate white light through light mixing, and at least three point-like light sources belonging to said second group of point-like light sources may have emission colors which generate white light through light mixing.

In the next place, the present invention can be applied to a display comprising a surface light source device and a displaying member illuminated by illumination light outputted from said surface light source device. According to the present invention, the surface light source device is any of the above surface light source devices.

The first light guide plate and the second light guide plate 5 employed in the present invention give effects as follows.

(1) Respective emission restraint regions restrain inner propagation light from being emitted before enough colour-mixing is done.

(2) Respective emission gradually-increasing regions cause emission of inner propagation to be promoted gradually as degree of colour-mixing increases.

(3) Respective emission promotion regions cause emission of inner propagation to be promoted after degree of colour-mixing has been heightened.

(4) Each light guide plates shows a brightness variation depending on distance from an incidence face (i.e. uniform brightness is obtained) due to overlapping, along a thickness direction of the first and second light guide plates, of each emission restraint regions and each emission promotion regions, and that of respective emission gradually-increasing regions.

In addition, since supply quantity of light from point-like light sources located at both ends of each point-like light source group is limited by light limiting means, the from the point-like light sources located at both ends from causing colour unevenness or colouring of emission.

Still further, a high-quality displaying is performed by a display employing the above surface light source device providing such a superior illumination output light as a illuminating means ror illuminating a displaying panel thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a display of an embodiment in accordance with the present invention, giving a cross section view along A-A shown in FIG. 2;

FIG. 2 is a plan view of a surface light source device employed in the embodiment in accordance with the present invention, showing state in which a liquid crystal display panel is removed from the display shown in FIG. 1;

FIG. 3 is a diagramic cross section view of the display of the embodiment in accordance with the present invention, wherein a frame and others are omitted;

FIG. 4 a is a diagramic partial plan view of the display shown in FIG. 2 for illustrating mounting state of a first light guide plate and LED package onto the frame, wherein some structure is omitted and a cross section of a part of the display is shown;

FIG. 4 b is a view as viewed from B-direction in FIG. 4 a;

FIG. 5 a is a cross section view along C-C in FIG. 4 a;

FIG. 5 b is a view as viewed from F1-direction in FIG. 5 a;

FIG. 6 a is a cross section view along D-D in FIG. 4 a;

FIG. 6 b is a view as viewed from F2-direction in FIG. 6 a;

FIG. 7 is a frontal view of spring means as viewed from E-direction in FIG. 6 a;

FIG. 8 is a partial plan view of a light guide plate for illustrating a state of propagation of light from a LED;

FIG. 9 is a diagram showing emission brightness curves, being illustration corresponding to a cross section of a main part of a surface light source device in accordance with the present invention;

FIG. 10 is a diagramic view of a surface light source device for showing chromaticity measurement points in the vicinity of a point-like light source unit where color unevenness or coloring of emission which would cause a problem;

FIG. 11 is a diagram illustrating x-chromaticity values (i.e. chromaticity values expressed by x-chromaticity-coordinates on CIE 1931 XYZ color model) at the respective chromaticity measurement points shown in FIG. 10 for the surface light source device of the embodiment and, comparatively, for a surface light source device according to a prior art (See Patent Document 2);

FIG. 12 is a diagram illustrating y-chromaticity values (i.e. chromaticity values expressed by y-chromaticity-coordinates on CIE 1931 XYZ color model) at the respective chromaticity measurement points shown in FIG. 10 for the surface light source device of the embodiment and, comparatively, for a surface light source device according to a prior art (See Patent Document 2);

FIG. 13 a is a partial perspective view of a frame employed in Modification 2 for the embodiment shown in FIG. 1, wherein a light guide plate and a point-like light source unit are omitted;

FIG. 13 b is a cross section view (mounting state illustration) along G1-G1, showing a state in which a light guide plate and a point-like light source unit have been mounted to the frame shown in FIG. 13 a;

FIG. 13 c is a cross section view (mounting state illustration) along G2-G2, showing a state in which a light guide plate and a point-like light source unit have been mounted to the frame shown in FIG. 13 a (mounting state illustration);

FIG. 13 d is a cross section view (mounting state illustration) along G3-G3, showing a state in which a light guide plate and a point-like light source unit have been mounted to the frame shown in FIG. 13 a (mounting state illustration);

FIG. 14 illustrates Modification 3 for the embodiment shown in FIG. 1, showing a state in which a LED package different from one employed in the embodiment shown in FIG. 1 has been mounted;

FIG. 15 is a diagramic cross section view of a surface light source device in accordance with a prior art;

FIG. 16 is a diagramic plan view of the surface light source device in accordance with the prior art; and,

FIG. 17 is a diagram illustrating a trouble state of a conventional surface light source device.

EMBODIMENT

Hereafter described in detail is on an embodiment in accordance with the present invention by referring to the drawings.

(Outlined Structure of Surface Light Source Device and Display Including the Same)

FIGS. 1 to 3 illustrate display 1 of the embodiment and surface light source device 2 included in display 1. FIG. 2 is a plan view of surface light source device 2. FIG. 1 is a cross section view along A-A shown in FIG. 2, wherein liquid crystal display panel 3 is added. FIG. 3 is a diagramic cross section view of display 1, wherein some parts thereof are omitted.

Liquid crystal display panel 3 (an example of member-to-be-illuminated) is backlighted by surface light source device 2. Surface light source device 2 comprises two light guide plates, namely, first light guide plate 4 and second light guide plate 5. Tease light guide plates 4, 5 are stacked. In FIG. 1, second light guide plate 5 is located above first light guide plate 4 (in this case, closer to liquid crystal display panel 3; in general cases, at an illumination light outputting side of surface light source device 2).

A side end face (minor face) of first light guide plate 4 provides incidence face (first incidence face) 6. The other side end face (minor face) of first light guide plate 4 provides distal end face (first distal end face) 5. On the other hand, a side end face (minor face) of second light guide plate 5 provides incidence face (second incidence face) 6. The other side end face (minor face) of second light guide plate 5 provides distal end face (second distal end face) 15.

Attention is to be paid to that incidence face 6 of first light guide plate 4 (first incidence face) and incidence face 6 of second light guide plate 5 (second incidence face) are located opposite to each other (at the left end of first light guide plate 4 and the right end of second light guide plate 5 in FIG. 1).

Referring to FIG. 1, an upper face (one major face) of first light guide plate 4 provides emission face (first emission face) 17 while lower face (the other major face) providing back face (first back face) 8. Needless to say, back face (first back face) 8 is located opposite emission face (first emission face) 17.

In a similar way, in FIG. 1, an upper face (one major face) of second light guide plate 5 provides emission face (second emission face) 11 while lower face (the other major face) providing back face (second back face) 18. Needless to say, back face (second back face) 18 is located opposite to emission face (second emission face) 11.

In addition, s thin low-refractive-index layer exists between emission face 17 of first light guide plate 4 and back face 18 of second light guide plate 5. It is noted that “ow-refractive-index layer” is a layer having refractive index lower than that of each of light guide plates 4, 5, being an air layer in a typical case. Further, reflection sheet 10 is disposed along back face 8 of first light guide plate 4 while first light control member 12 and second light control member 13 are disposed along emission face 11 of second light guide plate 5.

Point-like light source unit 7, 7 are disposed in the vicinity of incidence faces 6, 6 of first and second light guide plates 4, 5, respectively. As shown in FIG. 4 a, each of point-like light source units 7, 7 is provided with LEDs (point-like light sources) 14 (14 a, 14 b, 14 c). Emission color of LED 14 a is R (red), that of LED 14 b being G (green) and that of LED 14 c being B (blue).

As known well, white light is obtained by colour-mixing these three primary colours R, G, B. For each of point-like light source units 7, 7, LEDs 14 a, 14 b, 14 c are arranged in a line along a longitudinal direction of incidence faces 6, 6 (in X-direction in FIGS. 2, 4 a and 4 b).

It is noted that a point-like light source group (LEDs 14 a, 14 b, 14 c) supplying primary light toward incidence face 6 of first light guide plate 4 is called first point-like light source group (subsidiary ) point-like light source) and point-like light source group (LEDs 14 a, 14 b, 14 c) supplying primary light toward incidence face 6 of second light guide plate 5 is called second point-like light source group (primary light source).

It is noted that the two sets of (EDs 14 a, 14 b, 14 c) employed in the embodiment for each point-like light source group, as shown in FIG. 4 a. give merely an example. In general, one or more sets of point-like light source groups are arranged. In addition, emission colour combination of each point-like light source group is typically R, G, B, allowing other emission colour combination. Number of point-like light sources belonging to one point-like light source group is typically three, allowing two, four, five or more.

For example, in a case where three sets of point-like light source groups are arranged, a set consisting of (one R, one G, two Bs), another set consisting of (one R, two Gs, one B) and still another set consisting of (two Rs, one G, one B) may be employed. Still further saying, various variation regarding emission colour and number of point-like light sources may be adopted.

(First Light Guide Plate and Second Light Guide Plate)

First light guide plate 4 and second light guide plate 5 are made of a light permeable material such as polymethyl methacrylate (PMMA), polycarbonate (PC) or cycloolefin type resin. Each of first light guide plate 4 and second light guide plate 5 has a “generally rectangular (square)” plan shape, as shown in FIGS. 1, 4 a, and 4 b.

Incidence faces 6, 6 of first light guide plate 4 and second light guide plate 5 a plurality of positioning-projections 16, respectively. In other words, first light guide plate 4 and second light guide plate 5 have rectangular plan shapes of substantially the same size and shape, if positioning-projections 16 are removed. In addition, Each of first light guide plate 4 and second light guide plate 5 extends with uniform from thickness from each incidence face 6 to distal end face (another side face) 15.

First light guide plate 4 has emission restraint region (first emission restraint region) 20, emission promotion region (first emission promotion region) 21 and emission gradually-increasing region (first emission gradually-increasing region) 22 bridging these regions. In a similar way, second light guide plate 5 also has emission restraint region (second emission restraint region) 20, emission promotion region (second emission promotion region) 21 and emission gradually-increasing region (second emission gradually-increasing region) 22 bridging these regions.

Emission restraint region 20 of either of light guide plates 4 and 5 is formed in the vicinity of incidence face 6 and emission promotion region 21 of either of light guide plates 4 and 5 is formed far from incidence face 6. Emission gradually-increasing region 22 extends between both regions 20 and 21. Saying more concretely, back face 8, 8 of either of first light guide plate 4 and second light guide plate 5,

In each of first light guide plate 4 and second light guide plate 5, back face 8 or 18 is formed flat and smooth in a range of predetermined dimension (L1) from each incidence face 6 toward each distal end face 15, 15. This restrain intensively emission faces 17, 11 from emitting light in the range. In addition, the range provides emission restraint region.

It is noted that predetermined dimension L1 is set as “distance from incidence face 6 enough to cause light from LEDs 14 of R, G, B (14 a to 14 c) is sufficiently colour-mixed and become substantially white light”. AccordingLY, L1 is preferably larger than color-mixing beginning distance L1′ (L1>L1′). It is noted that color-mixing beginning distance L1′ is “distance from incidence face 6 enough to cause light from LEDs 14 of R, G, B (14 a to 14 c) substantially starts colour-mixing”.

If distance from incidence face 6, 6 of each of first light guide plate 4 and second light guide plate 5 exceeds L1′ mentioned above, colour-mixing progresses and degree of withe light generating increases. Colour-mixing beginning distance L1′ can be determined, for example, as follows.

Light from each LED 14 a, 14 b, 14 c . . . enters into light guide plate 4 or 5 through incidence face 6, becoming inner propagation light respectively. Now is provided that angular expansion of each inner propagation light in a plane parallel with emission face 11 is θ. In addition, provided is that arraying the respective pitches of LEDs of the same emission color are Pa, Pb, Pc. Distance between 14 a-14 a gives Pa, distance between 14 b-14 b gives Pb, and distance between 14 c-14 c gives Pc,

Under such situation, colour-mixing beginning distance L1′ can be defined as follow (See FIG. 8).

L1′=P/(2·tan θ);

where P is a value of the maximum of Pa, Pb, Pc.

Namely,

P=max[Pa,Pb,Pc]

If Pa=Pb=Pc, the following is obtained ((See FIG. 8).

L1′=Pa/(2·tan θ)=Pb/(2·tan θ)=Pc/(2·tan θ)

Thus, if any means for making angle θ large is applied to incidence face 6 or the vicinity thereof, relatively small L1′ can be employed, as compared with cases where such means is not applied.

It is preferable, however, that L1>L1′ is satisfied, as already mentioned, because L1′ is distance from incidence face 6 required for beginning of mixing of light of the same colour. It is noted that predetermined dimension L1 preferably has a valued adjusted as to be the optimum one under consideration of dimensions of first and second light guide plates 4, 5 and emission characteristics of LEDs 14 a to 14 c.

Now is provided that incidence face 6 of each of first light guide plate 4 and second light guide plate 5 has a dimension L2 along a longitudinal direction (X-direction), as shown in FIG. 8. If so provided, emission restraint regions 20, 20 of first light guide plate 4 and second light guide plate 5 are regions having area of L1)×(L2), respectively. Thus regions each having area of L1′)×(L2) can be regarded as colour-mixing regions.

Now is direction emission promotion region 21. First, emission promotion region (first emission promotion region) 21 of first light guide plate 4 is formed as to be overlapped with emission restraint region (second emission restraint region) 20 of second light guide plate 5. On the other hand, emission promotion region (second emission promotion region) 21 of second light guide plate 5 is formed as to be overlapped with emission restraint region (first emission restraint region) 20 of first light guide plate 4. In other words, respective ranges within predetermined dimension L1 from each distal end faces 15, 15 provide respective emission promotion regions 21, 21 for respective light guide plates 4, 5.

Emission region means is applied to areas in back faces 8, 18 corresponding to emission promotion region 21 in order to promote emission from emission faces 17, 11. Examples of employable emission region means are matted surface, numerous fine prismatic projection rows, blast-proceed surface, fine projections like pyramids or cones, rough surface with fine recesses and white-ink-printed surface. In general, any optional means may be employed so far as emission from emission faces 11, 17 is promoted.

Emission gradually-increasing regions 22, 22 are formed between respective emission restraint regions 20 and emission promotion regions 21 for respective light guide plates 4,5. Thus the following relation is satisfied for predetermined dimension L1.

2L1<distance from emission face 6 to distal end face 15 (total length of each light guide plate 4. 5).

Emission gradually-increasing region (firs emission gradually-increasing region) 22 of first light guide plate 4 and emission gradually-increasing region (second emission gradually-increasing region) 22 of second light guide plate 5 are overlapped with each other in a direction of thickness of first light guide plate 4 and second light guide plate 5.

It is noted that the above direction of thickness of first light guide plate 4 and second light guide plate 5 is called simply “thickness-direction” hereafter.

Each emission gradually-increasing region 22 is a region (bridging region) bridging each emission restraint region 20 and each emission promotion region 21. Respective emission gradually-increasing regions of light guide plates 4, 5 have emission promotion ability which increases according to distance from incidence face 6. In addition, each of light guide plates 4, 5 is formed as to avoid boundary between emission gradually-increasing region 22 and emission restraint region 20 from having discontinuous changing of emission promotion ability.

In a similar way, each of light guide plates 4, 5 avoids boundary between emission gradually-increasing region 22 and emission promotion region 21 from having discontinuous changing of emission promotion ability.

Such a gradient of emission promotion ability may be given to each emission gradually-increasing region 22 by changing (increasing) formation density of above-mentioned emission promotion means smoothly with distance from incidence face 6. It is noted that emission promotion means applied to each emission promotion region 21, 21 and that applied to each emission gradually-increasing region 22, 22 may be different from each other unless neither uncomfortable visual feeling nor unevenness in brightness as observed from above the surface light source device 2.

Next, now described are positioning-projections 16.

Rectangle-rod-like positioning-projections 16 are formed at an appropriate interval at portions between LEDs 14, 14 adjacent to each other on each incidence face 6 of each of light guide plates 4, 5 as shown in FIG. 4 a. These positioning-projections 16 can be fit into light-guide-plate-positioning-holes 24 formed on frame 23. Therefore, first light guide plate 4 and second light guide plate 5 is positioned not only regarding position in X-direction but also regarding Z-direction (up-down direction). As a result, both light guide plates 4, 5 is avoided from moving or allowed to move only by an extremely small distance in Z/direction. This extremely small distance is substantially equal to each gap formed between positioning-projection 16 and light-guide-plate-positioning-hole 24.

(First Light Control Member and Second Light Control Member)

The embodiment employs a light diffusion sheet as first light control member 12 and a prism sheet as second light control member 13. Structure, constituting material and basic functions of these optical members are well-known, being described very simply.

First light control member (light diffusion sheet) 12 is a sheet formed of a light permeable resin, having at least one roughened face. Light (here, white light) emitted from emission face 11 of second light guide plate 5 is diffused by first light control member 12, then being incident to second light control member 13. Such diffusion prevents emission promotion region 21 formed on each of light guide plates 4 and 5 from being conspicuous. It is noted that first light control member 12 has generally the same shape and dimension as those of emission face 11.

Next, the prism sheet disposed as second light control member 13 in the embodiment has an outer face (directed to liquid crystal display panel 3) on which a great number of prismatic projections 25 are formed (See FIG. 3). Prismatic projections 25 runs in a direction generally parallel to incidence face 6, 6 of each of light guide plates 2 and 5.

Second light control member 13 has a well-known light proceeding direction redirecting function by which proceeding directions of light diffused by first light control member 12 are gathered as to be near to a frontal direction and to be directed to liquid crystal display panel 3. Thus liquid crystal display panel 3 provides an increased brightness of displaying.

(Frame and Diaphragm Means)

As shown in FIGS. 1, 2, 4 a, 4 b to 6 a, 6 b, display 1 comprises box-like frame 23 opening largely toward upside. Frame 23 is formed of a resin material (such as PC), being provided with bottom plate 26. Piled up on bottom plate 26 are various members (reflection sheet 10, first light guide plate 4. second light guide plate 5, first light control member 12 and second light control member 13) for being accommodated. side Walls 27 to 30 standing up from bottom plate 26 restrain the various members accommodated inside from shifting (moving) along X-Y plane (parallel with a bottom plate plane).

In addition, side walls 27, 29 of frame 23 are opposite to incidence faces 6,6 of light guide plates 4, 5 respectively, being provided with windows 31 allowing light from LEDs 14 to passes through. Functions of these windows 31 are described below by seeing relation between first light guide plate 4 and frame 23.

As mentioned previously, groups of LEDs 14 are arrayed along the longitudinal direction of the respective incidence faces 6, 6 (i.e. X-direction in FIGS. 4 a, 4 b). CorrespondING to this, a plurality of windows 31 are formed. These windows 31 can be classified into windows corresponding to point-like light sources (14 a, 14 c) located at both end portions and the other windows. For the sake of convenience, the formers are denoted by reference numeral 30 (31 a) and the latter is denoted by reference numeral 30 (31 b).

Each window 30 (31 a) has “vertical length” smaller than “vertical length” of each window 30 (31 b). It is noted that “vertical length” is opening dimension along thickness direction. Namely, each window 30 (31 a) has upper and lower end portions 32, 33 each having a predetermined length as shown in FIG. 5 b for defining an opening. Accordingly, light from LEDs 14 (located at both end portions) corresponding to each window 30 (31 a) is subject to angularly narrowed expansion regarding thickness direction as compared with light from each of the other LEDs corresponding to each window 30 (31 b), causing incidence light quantity to incidence face 6 to be reduced (limited). In this sense, windows 30 (31 s) can be regarded as “light limiting means (incidence light quantity limiting means)”.

The reason why such light limiting means is arranged as to correspond to LEDs located at both end portions is that intensity of reflection light generated by light LEDs located at both end portions is to be reduced. That is, light from LEDs located at both end portions is more likely to be incident to side faces 34, 35 vertical to incidence face 6 after inner propagating within first light guide plate 4 as compared with light from the other LEDs. Such incident light becomes transmitting light and inner-reflection light. A large part of the transmitting light becomes again inner propagation light after being reflected by an inner surface of frame 23. Light paths made on the way of such processes, pass emission promotion region 21 doubtlessly, promoting emission on passing there. Such emission is apt to occur, in particular, in the vicinity of side faces 34, 35. It is noted that light emitted in such a way is likely to undergo insufficient emission colour-mixing with other LEDs, causing coloured emission or color unevenness to be induced. In other words, coloured emission or color unevenness tends to emerge in the vicinity of side faces 34, 35.

The above-described light limiting means, namely, windows 30 (31 a), can be effected as to restrain such phenomena. That is, quantity of the light which can be incident to incidence face 6 from LEDs located at both end portions. As a result, color unevenness and coloured emission are avoided from appearing. It is noted that “lateral dimension” (dimension along X-direction in FIGS. 4 a, 4 b) of each window 30 (31 a) providing light limiting means can be adjusted. Any way, opening area Sa of each window 30 (31 a) is smaller than opening area Sb of each window 30 (31 b). The optimum opening area ration Sa/Sb is preferably determined under consideration of various factors such as emission characteristics of LEDs, reflection characteristics of frame 23.

In addition, relation between first light guide plate 4 and windows 30 (31 b), located at other than both end portions, of frame 231 is as follows,

As shown in FIGS. 1 and 6 a, frame 23 covers edges upper and lower side along thickness direction (Z-direction) for first light guide plate 4 so as to avoid light from LEDs 14 (located at other than both end portions) from entering into first light guide plate 4 via edge 36 of the upper side or edge 37 of the lower side. Dimension of covering of each of edges 35, 37 by frame 23 is determined under consideration of shakable dimension of positioning-projections 16 of first light guide plate 4 within light-guide-plate-positioning-holes 24 of frame 23.

Thus light from LEDs 14 is avoided from entering into first light guide plate 4 via upper edge 36 or lower edge 37 even if first light guide plate 4 accommodated in frame 23 moves in a vertical direction (±Z-direction).

Further in the embodiment, since positioning-projections 16 of incidence face 6 of first light guide plate 4 is fit into light-guide-plate-positioning-holes 24 of frame 23, performed are not only positioning regarding thickness direction (Z-direction) but also positioning regarding longitudinal direction of incidence face 6 (X-direction), resulting in limited shaking movements. Therefore, light from LEDs 14 (located at other than both end portions) is avoided from entering into first light guide plate 4 via edge 36 of the upper side or edge 37 of the lower side even under using circumstance such that vibration or impact is effected.

As known well, “light entering into a light guide plate via such edges” can cause abnormal emission to occur locally. The emission is free from such local abnormal emission.

The above-described relation among frame 23, first light guide plate 4, respective windows 31 and others exists among frame 23, second light guide plate 5, respective windows 31 and others, in a similar manner. Therefore, repeated description is omitted.

Now is added some description on frame 23 employed in the embodiment. Frame 23 is formed of black resin material. This causes light from LEDs 14 to be avoided from entering into light guide plates 4, 5 through portions other than incidence faces 6, 6 by light absorbing. In addition, this shading function assists functions of light limiting means.

It is noted, however, that materials which have any shading function and any colour other than black may be employed depending on emission characteristics of LEDs 14 and others.

(State of LED-package Mounting to Frame)

As shown in FIGS. 1, 5 a and 6 a, point-like light source units 7 are LED-packages 41 fit into package-engaging-recesses 40 formed on frame 23.

LED-packages 41 are pressed to first light guide plate 4 or second light guide plate 5 together with thermal conductive sheets 38 via thermal conductive sheets 38 by means of spring means 2.

Thus LED-packages 41 are prevented from escaping from package-engaging-recesses 40 of frame 23, being secured at certain positions.

It is noted that LED-packages 41 pressed by spring means 42 push thin upper edge vicinity portions 43 a, 43 b and thin lower edge vicinity portions 44 a, 44 b of windows 31 of frame 23 to incidence face 6 of first light guide plate 4 or second light guide plate 5.

FIG. 7 is a frontal view of spring means 42 as viewed from E-direction in FIG. 6 a. As shown in FIG. 7, spring means 42 is produced by applying sheeting processes to a plate-like elastic member. Spring means 42 evenly presses a plurality of portions (a plurality of portions corresponding to LED-packages 41 in the embodiment) of point-like light source unit(s) 7 securing a plurality of LED groups as being unified.

Spring means 42 include elastic contacting elements 45 pressing point-like light source units 7. Elastic contacting elements 45 are formed by partially cut-and-raising plate-like side walls of spring means 42. It is noted that thermal conductive sheets 38 may be omitted in the embodiment. If so, LED-packages 41 may be fit into package-engaging-recesses 40 so that point-like light source units 7 may be directly pressed by elastic contacting elements 45.

(Effects and Advantages of the Embodiment)

FIG. 9 is a diagram showing an emission brightness curve (solid line A=first pattern) under the following lighting condition 1, an emission brightness curve (solid line B=second pattern) under the following lighting condition 2 and an emission brightness curve (solid line C=third pattern) under the following lighting condition 3 for surface light source device 2, being illustration corresponding to a cross section of surface light source device 2.

-   -   Lighting condition 1; Lighted on are only respective LEDs 14 (14         a to 14 c) belonging to point-like light source unit 7 close to         incidence face 6 of first light guide plate 4.     -   Lighting condition 2; Lighted on are only respective LEDs 14 (14         a to 14 c) belonging to point-like light source unit 7 close to         incidence face 6 of second light guide plate 5.     -   Lighting condition 3; Lighted on are respective LEDs 14 (14 a to         14 c) belonging to respective point-like light source units 7         close to incidence faces 6 of first and second light guide         plates 4 and 5.

The followings are understood from FIG. 9.

(a) Seeing the first pattern, emission restraint region (first emission restraint region) 20 of first light guide plate 4 provides emission little.

(b) Seeing the second pattern, emission restraint region (second emission restraint region) 20 of second light guide plate 5 provides emission little.

(c) Seeing the first pattern, emission promotion region (first emission promotion region) 21 of first light guide plate 4 provides generally even emission. Seeing in detail, found is an extremely gentle reduction in brightness toward distal end face 15.

(d) Seeing the second pattern, emission promotion region (second emission promotion region) 21 of second light guide plate 5 provides generally even emission. Seeing in detail, found is an extremely gentle reduction in brightness toward distal end face 15.

(e) Seeing the first pattern, emission gradually-increasing region (first emission gradually-increasing region) 22 of first light guide plate 4 gives brightness increasing as approaching distal end face 15.

(f) Seeing the second pattern, emission gradually-increasing region (second emission gradually-increasing region) 22 of second light guide plate 5 gives brightness increasing as approaching distal end face (g) Seeing the third pattern, generally constant emission occurs overall. Seeing in detail, found are extremely gentle reductions in brightness from a center portion toward distal end face 15 of first light guide plate 4 and distal end face 15 of second light guide plate 5.

Judging from the above matters, surface light source device 2 of the embodiment provides highly even brightness.

Next described is colour uniformity of illumination output light of surface light source device 2 in accordance with the embodiment. FIG. 10 is a diagramic plan view of surface light source device 2. Shown in the illustration are chromaticity measurement points (1 to 13) for emission, the chromaticity measurement points being located in the vicinity of point-like light source units (in a range within about 20 mm from incidence face 6 of surface light source device 2 of 8 inch size).

FIG. 11 is a diagram illustrating x-chromaticity values obtained at the respective chromaticity measurement points (1 to 13) shown in FIG. 10 for surface light source device 2 of the embodiment and, comparatively, for surface light source device according 102 of a prior art (See FIG. 17).

In addition, FIG. 12 is a diagram illustrating Y-chromaticity values obtained at the SAME respective chromaticity measurement points (1 to 13) for surface light source device 2 of the embodiment and, comparatively, for surface light source device according 102 of a prior art (See FIG. 17). It is to be noted that surface light source device 102 of a prior art is not provided with light limiting means for adjusting light incidence to the light guide plate from both end LEDs 14.

In addition, “x-chromaticity value” is chromaticity value expressed by x-chromaticity-coordinate on CIE 1931 XYZ color model. in a similar way, “y-chromaticity value” is chromaticity value expressed by y-chromaticity-coordinate on CIE 1931 XYZ color model.

As shown these figures, surface light source device 2 of the embodiment (See curves denoted by PRESENT INVENTION in FIGS. 11, 12) shows, roughly saying, symmetry with respect to center measurement point 7. Further saying, unevenness in x-chromaticity values and y-chromaticity values a found between center measurement point 7 and both end measurement points 1, 13 is very small.

To the contrary, conventional surface light source device 102 (See curves denoted by PRIOR ART in FIGS. 11, 12) fails to show symmetry with respect to center measurement point 7. Besides, unevenness in chromaticity values is large.

As described above, the embodiment provides illumination light with evenness in chromaticity values higher than that of the prior art. It is a significant merit that vicinity of side faces 34, 35 of light guide plates 4, 5 has reduced colour unevenness and restrained colouring. Further, this merit makes high quality color displaying easy.

It is noted that the present invention is not limited by the embodiment, allowing modifications as follows.

(Modification 1)

Below noted are examples of means employable instead of light limiting means which is provided by limiting opening area of windows 31 (31 a) of frame 23.

(i) Windows 31 of frame 23 may have the same opening area under a condition such that shading members (such as shading films or shading tapes), not shown, may be applied to emission surfaces of both end LEDs 14, 14 for shading a predetermined area of the surfaces of both end LEDs 14, 14. That is, shading members may used as shading means.

(ii) Shading members may be applied to portions, opposite to both end LEDs 14, 14, of incidence faces 6 of first light guide plate 4 and second light guide plate 5.

(iii) Windows 31 of frame 23 may have the same opening area under a condition such that spacers, not shown, each having light limiting opening with opening areas each smaller than opening area of window 31 of frame 23 may be disposed between both end LEDs 14, 14 and first and second light guide plate 4,5.

(iv) Instead of the spacers, light transmissivity adjusting sheets (such as diffusion sheets) may be disposed as light limiting means.

(v) Windows 31 of frame 23 may have the same opening area under a condition such that shading ink may be applied to emission surfaces of both end LEDs 14, 14 for providing shading means.

(vi) Windows 31 of frame 23 may have the same opening area under a condition such that shading ink may be applied to part areas of incidences face 6 at locations corresponding to both end LEDs 14, 14 for providing shading means.

(Modification 2)

FIGS. 13 a to 13 d illustrate Modification 2 of the above embodiment. This modification employs frame 53 formed of a metal plate instead of frame 23 formed of a resin material. As shown in FIGS. 13 a to 13 d, Frame 53 has windows 54 which are formed at portions opposite to LED-packages 41 of point-like light source units 7. Between windows 54, 54, nails 55, 56 are arranged for securing point-like light source units between LED-packages 41, 41.

First nails 55 abutting an upper portion of point-like light source units 7 and second nails 56 abutting a lower portion of point-like light source units 7 are arranged alternately.

In addition, frame 23 uses holes provided by first and second nails 55, 56 as light-guide-plate-positioning-holes 57 into which positioning-projections 58 of first light guide plate 4 and second light guide plate 5 are engaged.

Further, each window 54 of frame 53 has upper edge vicinity 60 and lower edge vicinity 61 which shade upper edge 36 and lower edge 37 of each of each incidence face 6 of light guide plates 4, 5 as to avoid light of LEDs 14 from being incident thereto.

In this modification, it is preferable that frame 53 is formed of a highly reflective metal or has a frame inner surface, on which first light guide plate 4 and second light guide plate 5 are put and to which any means for heightening light reflectivity. If so, light utilization efficiency is heightened and more bright and uniform white light illumination is provided.

(Modification 3)

FIG. 14 illustrates a state in which LED packages 62 different from LED-packages 41 of the embodiment. Each LED package 62 employed in this modification has a hemisphere emission surface in contrast with each LED-package 41, employed in the embodiment, having a flat emission surface. Such LED-packages 62 are not able to push thin portions around windows 31 toward the sides of first light guide plate 4 and second light guide plate 5. Therefore, first light guide plate 4 and second light guide plate 5 are pressed to the side of windows 31 of frame 23 with LED-packages 62. Alternatively, opening areas of windows are determined under consideration of gaps between frame 23 and first and second light guide plates 4, 5.

r Upper edges 36 and lower edges 37 of incidence faces 6 of first light guide plate 4 and second light guide plate 5 are prevented from receiving light from LEDs 14. Surface light source devices 2 each employing LED-packages 62 in accordance with this modification have merits similar to those of the embodiment.

(Other Modifications)

Other modifications are employable as follows.

(I) Second light control member 13 may be a prism sheet directed downward (prismatic projections are formed as to correspond to second light control member 12). In addition, on this downward prism sheet, and the prism sheet having prismatic projections running in a described vertical to the longitudinal direction of incidence face 6 may be disposed as a third light control member.

In general, numbers of prism sheets and running directions and shapes of prismatic projections are can be determined optionally in designing. Regarding light diffusion sheet employed as third light control member, no particular limitation of numbers is not required.

A light diffusion sheet may be disposed outside of second light control member 13.

(III) In general, kinds and numbers of light control members disposed on emission face 11 are not limited specifically. For example, light control members disposed on emission face 11 may include a polarization separating sheet. It is noted that a polarization separating sheet is a well-known optical member for utilizing only a required polarization component as emission.

(IV) Emission promotion means may be applied to emission faces 17, 11 instead of portions other than back faces 8, 18, or in addition to back faces 8, 18.

(V) The above embodiment employs point-like light source unit 7 each having a plurality of sets of LEDs 14 a, 14 b, 14 c of R, G, B. However, this does not limit the scope of the present invention. For example, LEDs of emission colors other than R, G, B, as required.

(VI) The present invention allows each group of point-like light sources to have somewhat unevenness in emission color (emitting roughly white light). Reflection sheet 10 may be omitted. In particular, if frame 23 has a light reflective bottom inner face, omitting of reflection sheet 10 brings no problem.

(VII) Positioning-projections 16 of first light guide plate 4 and second light guide plate 5 may be shaped like round rods. In general, light-guide-plate-positioning-holes 24 may be round holes allowing positioning-projections 16 to be fit into.

(VII) Thickness of light guide plates 4, 5 may decrease with approaching each distal end face 15 from each incidence face 6.

(IX) Displaying members (member-to-be-illuminated) irradiated by surface light source devices 2 may be other than LCD-panel 3, for example, may be transmitting plate on which an image is illustrated.

(X) In the specification and drawings, “up” and “down” are used only for the sake of description, corresponding to Z-direction in FIG. 1. It is to be noted that, for example, if surface light source device 2 is used under an oblique installing condition, Z-direction is also inclined. 

1. A surface light source device comprising: a first light guide plate; a first group of point-like light sources including at least three point-like light sources of emission colors different in emission color; a second group of point-like light sources including at least three point-like light sources of emission colors different in emission color; said first light guide plate having a first incidence face provided by a side end face, a first distal end face opposite to said first incidence face, a first emission face provided by a major face and a first back face opposite to said first emission face; said second light guide plate having a second incidence face provided by a side end face, a second distal end face opposite to said second incidence face, a second emission face provided by a major face and a second back face opposite to said second emission face; said first light guide plate and said second light guide plate overlapping each other so that said first emission face and said second back face are disposed face to face across a low-refractive-index-layer, which is interposed between them and has a refractive index lower than those of said first light guide plate and said second light guide plate, and said first incidence face and said second incidence face are located at different sides with respect to said first light guide plate and said second light guide plate; point-like light sources belonging to said first group of point-like light sources being disposed along said first incidence face at a predetermined interval; and, point-like light sources belonging to said second group of point-like light sources being disposed along said second incidence face at a predetermined interval, wherein said first light guide plate is provided with a first emission restraint region, a first emission gradually-increasing region and a first emission promotion region arranged in order away from said first incidence face toward said first distal end face; said second light guide plate is provided with a second emission restraint region, a second emission gradually-increasing region and a second emission promotion region arranged in order away from said second incidence face toward said second distal end face; said second emission restraint region overlaps with said first emission promotion region in a thickness-direction of said first light guide plate and said second light guide plate; said second emission promotion region overlaps with said first emission restraint region in the thickness-direction of said first light guide plate and said second light guide plate; said second emission gradually-increasing region overlaps with said first emission gradually-increasing region in the thickness-direction of said first light guide plate and said second light guide plate; emission promotion ability in said first emission gradually-increasing region increases gradually and smoothly away from said first emission restraint region toward said first emission promotion region; emission promotion ability in said second emission gradually-increasing region increases gradually and smoothly away from said second emission restraint region toward said second emission promotion region; a first limiting means is disposed between said first incidence face and each of point-like light sources which are both-end-located point-like light sources of said first group of point-like light sources and are located at both ends of said first group of point-like light sources respectively, in order to limit light quantity incident to said first incidence face from said both-end-located point-like light sources of said first group of point-like light sources; and, a second limiting means is disposed between said second incidence face and each of point-like light sources which are both-end-located point-like light sources of said second group of point-like light sources and are located at both ends of said second group of point-like light sources respectively, in order to limit light quantity incident to said second incidence face from said both-end-located point-like light sources of said second group of point-like light sources.
 2. A surface light source device in accordance with claim 1, wherein at least three point-like light sources belonging to said first group of point-like light sources have emission colors which generate white light through light mixing; and at least three point-like light sources belonging to said second group of point-like light sources have emission colors which generate white light through light mixing.
 3. A display comprising: a surface light source device; and a display panel irradiated by illumination light outputted from said surface light source device, wherein said surface light source device is in accordance with claim
 2. 4. A display comprising: a surface light source device; and a display panel irradiated by illumination light outputted from said surface light source device, wherein said surface light source device is in accordance with claim
 1. 